Heretofore, gate valve structures for use in fluid distribution systems having a corrosive promoting fluid flowing therethrough, such as water or the like, and utilizing a non-rising valve stem did not provide means to seal the threads of the valve stem from the fluid when the gate member of the valve was in the fully opened position. With fluids such as water, the presence of oxygen in the water acted as a corrosive catalyst and, consequently, the threads of the valve stem became corroded in time and, thus, increased the torque necessary to close the valve or subsequently open the valve. More recently, gate valve structures have been designed to provide a corrosion resistant coating on the threads of the valve stem or in some instances to provide sealing means between the gate member and the valve stem which would prevent fluid from coming into contact with the threads of the stem when the gate member of the valve was in the fully opened position. The latter-mentioned of the two above arrangements usually required the entire gate member or at least a portion thereof surrounding the valve stem to be coated with a resilient material, which resilient material would engage an interior surface of the valve casing or the under surface of the thrust collar of the valve stem in the fully opened position. Even more recently, gate valve structures have been designed wherein the threads on the gate member and on the valve stem have been protected against corrosion by sealing means, which sealing means was effective in any position of the valve, including and between fully closed position and fully opened position. This latter-mentioned type of gate valve structure was costly to manufacture and required the valve stem to be provided with an upper portion having interior threads and a lower portion carried within the hollow gate member and having exterior threads cooperating with the interior threads of the upper portion.
Gate valve structures used in water distribution systems and in other fluid distribution systems wherein the fluid can cause corrosion to the interior of the valve structure and its working parts are generally not operated frequently. In other words, a gate valve in a water distribution system, once the system has been completed, is usually opened and left open for great lengths of time until it is necessary to repair the system downstream of the gate valve structure or it is necessary to shut off water downstream of the same to a particular point of use. As mentioned above, the presence of oxygen in the water acts as a corrosive catalyst and aids in the formation of corrosion of the internal parts of the gate valve and it is necessary to provide a positive means to prevent the water from engaging the threads on the valve stem when the valve is fully open. When the valve is fully open, there is the greatest passage of fresh water through the valve and, consequently, it is at this time the stem threads on the valve should be protected. When the gate valve structure is closed, there will be no flow of water through the valve structure with the gate member making a seal with the valve seat and, thus, sealing off the valve actuating mechanism to prevent corrosion. Of course, when the valve begins to open, there is some water which engages the valve stem, but, of course, the valve stem is receiving the gate member and, consequently, it is only necessary to provide a seal when the gate member gets into the fully opened position. This arrangement, even though the valve is not operated frequently, will make the valve operation easier as it prevents corrosion.